The invention relates to a multi-flame burner with burner heads and accompanying connection pipes, which are set up to generate at least one burner flame directed along a respective flame axis when supplied with a fuel, as well as to a method for preheating a workpiece, in particular a pipe or large-diameter pipe, with a corresponding multi-flame burner.
Workpieces must be preheated in a very wide range of material handling and manufacturing processes. The present invention here draws reference first and foremost to preheating in welding operations. However, the invention can basically also be used in other methods in which introducing heat by means of flames is desired.
In known welding processes, the workpieces to be welded are preheated for various reasons. The primary danger associated with a lack of preheating lies in the so-called hydrogen embrittlement in the heat affected zone. In addition, transformable steels there tend to exhibit an increased hardness at elevated cooling rates of the kind encountered without preheating. This yields an increased tendency toward cold crack formation in the joining region of the workpiece. For example, these disadvantages become even greater while welding pipes with an increasing pipe diameter. For this reason, preparing large-diameter pipes via preheating for a subsequent welding operation represents a preferred application for the present invention.
The achievable hardness, and hence the danger of cold cracks, here essentially depends on the thickness of the workpiece to be machined, two- or three-dimensional heat dissipation, present alloy elements and their contents, the respective heat introduced in the welding operation and/or the component temperature. The latter can be influenced by preheating. Preheating reduces the cooling rate of the seam region, and improves hydrogen effusion. In addition, it has a favorable effect on the residual welding stress state of the welded joint.
The necessary preheating temperature can be respectively ascertained, and the heat to be introduced and/or the use of respective preheating device can be determined based hereupon. In particular the basic tenacity of the material, its wall thickness, the seam shape, the used welding technique and welding speed are to be considered when determining the preheating temperature.
Preheating is basically required at processing temperatures of under 5° C. and when specific thickness limits are exceeded (especially for high-strength steels). For example, the preheating temperature measures 80 to 200° C.
As a rule, workpieces are preheated prior to welding preferably using multi-flame burners, which for reasons explained in greater detail below are operated with acetylene and oxygen-containing fuel mixtures or acetylene and compressed air-containing fuel mixtures, for example. Preheating takes place to prepare a region proximate to the welded seam on the workpiece (e.g., pipe or large-diameter pipe) for introducing a welded seam in this preheated region, which is hence already warm at the start of the welding process (warm here means at least warmer than the ambient air).
In particular when preheating large-diameter pipes, concentrated heat must be introduced into the region close to the welded seam, without melting the workpiece surface and leaving behind residual moisture from the exhaust gas of the flame. However, the large amount of hot exhaust gases that arises given a concentrated introduction of heat places a burden on the burner itself, depending on burner configuration. This can end up thermally damaging the used burner heads. This applies in particular given improper operation and adjustment. Appropriate safety precautions are expensive.
In this conjunction, pipes with a diameter ranging from 1 to 12 meters are designated as large-diameter pipes.
Therefore, the need exists for improved ways of heating workpieces, in particular pipes and large-diameter pipes, which do not exhibit these disadvantages and can be safely operated.